Large wind shift on the great plains during the medieval warm period

Spring-summer winds from the south move moist air from the Gulf of Mexico to the Great Plains. Rainfall in the growing season sustains prairie grasses that keep large dunes in the Nebraska Sand Hills immobile. Longitudinal dunes built during the Medieval Warm Period (800 to 1000 years before the present) record the last major period of sand mobility. These dunes are oriented NW-SE and are composed of cross-strata with bipolar dip directions. The trend and structure of the dunes record a drought that was initiated and sustained by a historically unprecedented shift of spring-summer atmospheric circulation over the Plains: Moist southerly flow was replaced by dry southwesterly flow.     

Postglacial change in sea level in the Western north atlantic ocean

Radioactive carbon determinations of the age of peat indicate that at Bermuda, southern Florida, North Carolina, and Louisiana the relative sea level has risen at approximately the same rate, 2.5 x 10(-3) foot per year (0.76 x 10(-3) meter per year), during the past 4000 years. It is proposed tentatively that this is the rate of eustatic change in sea level. The rise in sea level along the northeastern coast of the United States has been at a rate much greater than this, indicating local subsidence of the land. Between Cape Cod and northern Virginia, coastal subsidence of 13 feet appears to have occurred between 4000 and 2000 years ago and has continued at a rate of about 1 x 10(-3) foot per year since then. On the northeastern coast of Massachusetts, subsidence of 6 feet occurred between 4000 and 3000 years ago; since then sea level has risen at about the eustatic rate. Between 12,000 and 4000 years ago, sea level rose at an average of about 11 x 10(-3) foot per year. The part played by local subsidence or temporary departures from the average rate during this period is uncertain.     

农业起源——中纬度地区冰后期贮藏行为的产物

农业起源是新石器时代的产物,但是这一过程以什么为标志,即驯化与种植开始于什么行为?在有关农业起源问题的探讨中,没有明确地说明。农业起源的过程实际上是驯化的植物种质变化的起源与人类驯化行为起源的结合体,两者缺一不可。前者在后来表现出与野生状态的差别,后者则带来工具的进步。由于气候变化是新旧石器之交的关键因素,而农业是其产物,而能够在其中找到直接的决定因子的是贮藏行为,并且只有在中纬度地区,贮藏才能诱发种植与驯养。因为高纬度地区人类无法生存,低纬度地区人类不需要贮藏。中纬度地区因为气温因素,导致人类必须依赖于贮藏度过采集食物的缺乏期。贮藏诱发了种植;贮藏参与了驯化,贮藏产生了农作物。动物的贮藏行为导致了驯养,家畜因此而起源。     

Calibration of the great american interchange

From radioisotopic (potassium-argon) age determinations of tuffs and magnetostratigraphy of Late Tertiary mammal-bearing beds in Catamarca Province, northwest Argentina, refined estimates have been obtained for the durations and boundaries of beds of Chasicoan (Middle Miocene) through Chapadmalalan (Pliocene) age. An age of 9.0 million years is tentatively accepted for the Chasicoan-Huayquerian boundary, 5.0 million years for the Huayquerian-Montehermosan boundary, and 3.0 million years for the Montehermosan-Chapadmalalan boundary. Procyonids (raccoons and their allies), a group of North American origin, are first recorded in South America in a level immediately below a unit dated at 6.0 million years. Cricetine rodents of the tribe Sigmodontini are first recorded in South America in beds of Montehermosan age in Argentina. Ground sloths, a group of South American origin, first appear in North America in Early Hemphillian time in beds dated between 9.5 and 9.0 million years. The Panamanian land bridge was established by 3.0 million years ago, and an interchange of the terrestrial faunas was well under way by Late Blancan time (around 2.5 million years before present) in North America and by Chapadmalalan time in South America.     

Natural displacement of pollution from the great lakes

A simplified mathematical model of a lake system indicates that, if the pollution of the Great Lakes were discontinued, the natural flow through the lower Great Lakes would be sufficient to remove about 90 per cent of the waste in about 20 years. On the other hand, hundreds of years would be required to displace the pol lution from Lake Michigan and Lake Superior.     

Mammalian evolution and the great american interchange

A reciprocal and apparently symmetrical interchange of land mammals between North and South America began about 3 million years ago, after the appearance of the Panamanian land bridge. The number of families of land mammals in South America rose from 32 before the interchange to 39 after it began, and then back to 35 at present. An equivalent number of families experienced a comparable rise and decline in North America during the same interval. These changes in diversity are predicted by the MacArthur-Wilson species equilibrium theory. The greater number of North American genera (24) initially entering South America than the reverse (12) is predicted by the proportions of reservoir genera on the two continents. However, a later imbalance caused by secondary immigrants (those which evolved from initial immigrants) is not expected from equilibrium theory.